This invention relates in general to low pressure casting machines and in particular to an apparatus and process for automatic refilling of a low pressure casting machine.
Many vehicle components, including wheels and full face wheel discs are cast in permanent multi-piece molds from alloys of light weight metals, such as aluminum, magnesium and titanium. Due to the large volume of parts needed, the casting processes are highly automated. Two frequently utilized casting processes are gravity-poured and low pressure casting. Gravity-poured casting involves pouring a charge of molten metal into a mold cavity. Gravity causes the molten metal to fill the mold cavity while gases within the mold cavity are forced out of mold vents. Riser cavities are formed in the upper portions of the mold cavity to assure that the mold cavity is completely filled and to feed additional molten metal into the mold to compensate for metal shrinkage as the casting cools and solidifies. After the casting is removed from the mold, the risers are removed and re-melted.
Low pressure casting involves forcing molten metal into the cavity mold under a low pressure, which, for casting wheel components, is typically within a range from approximately six psi to approximately seven psi. The pressure assures that the mold cavity is completely filled with molten metal. Typically, because the pressure force-fills the mold, the size of the riser cavities can be reduced, or the riser cavities may be eliminated entirely. Accordingly, less molten metal is needed for each casting and the need to re-melt the risers is reduced or eliminated. Additionally, low pressure casting facilitates casting of more complex shapes since the pressure forces the molten metal into all the portions of the mold cavity. Also, low pressure casting usually runs at lower mold temperatures and shorter cycle times than conventional gravity-poured casting processes. The rapid solidification rates associated with low pressure casting processes result in castings with finer grain size, smaller spacing between dendrite arms and enhanced mechanical properties.
Referring now to the drawings, there is illustrated in FIG. 1 a typical known low pressure casting machine 10. The machine 10 includes a thermally insulated lower chamber 11 which holds a pool of molten metal 12. A filler tube 13 extends through a side of the lower chamber 11 from above the maximum height of the metal pool 12. A removable filler cap 14 is mounted upon the upper end of the filler tube 13. When closed, the filler cap 14 forms an air-tight seal for the lower chamber 11. A pressurization port 14A is formed through the lower chamber walls to allow pressurization of the chamber 11. Similarly, a vent port 14B is formed through the lower chamber walls to allow venting of the chamber 11.
A multi-piece mold 15 is typically mounted above the lower chamber 11; however, the chamber 11 is not necessarily positioned below the mold 15. As illustrated in FIG. 1, the mold 15 includes an upper member 16 which cooperates with a lower member 17 to form a mold cavity 18. For simplicity, a two piece mold 15 is illustrated in FIG. 1. A hollow fill tube, or stalk, 19 extends from the mold 15 and through the top of the casting machine""s lower chamber 11. The lower end of the stalk 19 extends into the pool of molten metal contained within the chamber 11. The upper end of the stalk 19 extends through the lower mold member 17 and communicates with the mold cavity 18. The upper mold member 16 is attached to a conventional mechanism 20 which raises the upper member 16 to allow removal of the casting from the mold cavity 18. Once the casting is removed from the mold cavity 18, the mechanism 20 is reversed to lower the upper member 16 and re-close the mold 15.
To operate the low pressure casting machine 10, the filler cap 14 is opened and molten metal poured into the filler tube 13 to fill the lower chamber 11 to the level of the lower end of the filler tube 13. The filler tube 13 is then cleared of molten metal and the filler cap 14 closed and sealed. The mold 15 is closed and a pressurized gas is introduced through the pressurization port 14A into the lower chamber 11. The pressurized gas forces molten metal up the stalk 19 and into the mold cavity 18, as shown by the small arrows in FIG. 1. The pressure is maintained while the molten metal in mold cavity 18 solidifies into a casting for a component. The pressurized gas is then vented from the lower chamber 11 through the vent port 14B and the upper mold member 16 is raised to open the mold 15 opened for removal of the casting from the mold cavity 18. The upper mold member 16 is lowered to close the mold 15 and the lower chamber 11 re-pressurized to cast another component.
After each casting, the level of molten metal remaining in the lower chamber 11 is reduced. Accordingly, after a number of components are cast, the filler cap 14 is re-opened and the chamber 11 refilled with molten metal. The molten metal is typically manually transported to the individual casting machine from a centrally located refractory furnace in a thermally insulated covered ladle. Because the lower end of the filler tube 13 is some distance above the surface of the remaining metal when refilling occurs, the molten metal being added to the lower chamber 11 tends to cascade into the chamber 11.
This invention relates to an apparatus and process for automatic refilling of a low pressure casting machine.
The prior art low pressure casting machine described above has a number of disadvantages. While the lower chamber is being refilled, the casting machine is out of service, which results in a loss of production and loss of mold temperature. Refilling the lower chamber of the casting machine interrupts the production process, causing process variability and corresponding product quality problems. The transport of molten metal to the casting machine for refilling the lower chamber is labor intensive. Because refilling requires pouring molten metal from a ladle and though a filler tube into the lower chamber, there is a chance that some of the molten metal could be spilled. Accordingly, the refilling process is hazardous. Manual refilling of the lower chamber is the last barrier to a fully automated low pressure casting process.
The agitation and turbulence caused by molten metal cascading from the filler tube into an almost empty lower chamber is conductive to generating oxidation byproducts and air bubbles which can be entrapped in the molten metal. When the oxidation byproducts and air bubbles are then cast into the component, the resulting product may have to be scrapped. Such raw material level variations are a source of process variability introduced by the traditional manual refilling process for the lower chamber.
Accordingly, it would be desirable to provide an apparatus for automatically refilling the lower chamber of a low pressure casting machine.
The present invention contemplates a low-pressure casting machine having a thermally insulated chamber adapted to retain a supply of molten metal and a mold having a cavity which communicates with the chamber. The machine also includes a pressurization device communicating with the chamber which is selectively operative to supply a pressurized gas to the chamber to force molten metal from the chamber into the mold cavity. The machine further includes a fill line communicating with the chamber, the fill line being operative to supply molten metal into the chamber. A normally closed fill valve is disposed in the fill line. The fill valve is selectively operative to control the flow of molten metal through the fill line and into the chamber.
In the preferred embodiment, the fill valve is actuated by a solenoid and the fill line communicates with a refractory furnace which supplies molten metal to the fill line. Gravity urges the molten metal from the furnace to the chamber. The fill line can be sloped, or the furnace can be at a higher elevation than the thermally insulated chamber, to enhance the flow of molten metal through the fill line.
Additionally, the casting machine includes a vent which communicates with the chamber. The vent is selectively operable to release the pressurized gas from within the chamber. The casting machine further includes a mechanism attached to a portion of the mold which selectively opens and closes the mold. A level sensor is optionally mounted within the thermally insulated chamber. The sensor is coupled to the fill valve solenoid and is operative, upon said molten metal within the thermally insulated chamber reaching a predetermined level, to cause the fill valve to close.
It is further contemplated that the casting machine is one of a plurality of casting machines. The casting machines are connected by associated fill lines to the refractory furnace. Each of the fill lines includes a fill valve which is selectively operable to control the flow of molten metal from the refractory furnace to the casting machines.
The invention also contemplates a process for casting a metal component which includes providing the above described casting machine. The chamber is pressurized to force molten metal from the chamber into the mold cavity. The molten metal in the mold cavity is allowed to solidify to form a casting. The chamber is then depressurized. The mold and the fill valve are opened to remove the casting and to allow molten metal to flow into the chamber to replace the molten metal used to form the casting. After the casting is removed from the opened mold, the mold and fill valve are closed.
The process further includes, prior to pressurizing the chamber, opening the fill valve to initially fill the chamber with molten metal and allowing the temperature of the casting machine to stabilize. In the preferred embodiment, the mold is for casting a vehicle wheel component.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.